1. Field of the Invention
The present invention belongs to a technological field related to magnetic resonance imaging systems utilized for in-vivo imaging diagnosis, and relates to magnet systems utilized in magnetic resonance imaging systems. In particular, the present invention relates to enhancement of magnetic-field homogeneity in a homogeneous static-magnetic-field space region generated by a magnet system.
2. Description of the Related Art
Magnetic resonance imaging systems are roughly categorized in accordance with the shape of a magnet system into a cylindrical type and a facing type; in recent years, the facing type has become dominant. Facing-type magnetic resonance imaging systems form a sphere-shaped homogeneous static-magnetic-field space region between a pair of magnet bodies, and are superior to cylindrical-type systems in terms of the subject's feeling of not being confined and convenience for diagnosis-related staffs to walk around during a diagnosis. The strength of the magnetic field of a homogeneous static-magnetic-field space region in a facing-type magnetic resonance imaging system is 0.2 to 1.0 tesla, and a tolerance within a virtual sphere is normally several ppm. In addition, the size of a facing-type magnetic resonance imaging system is, for example, approximately 2 to 3 m in height and, in a plan view, approximately 2 m in maximal diameter. The weight of a facing-type magnetic resonance imaging system depends on the method of shielding magnetic field; for example, in the case where no magnet body is utilized, the weight is approximately 10 t.
In facing-type magnetic resonance imaging systems as described above, it is important to secure the magnetic-field homogeneity of a homogeneous static-magnetic-field space region, while enhancing downsizing and weight saving of the system. In conventional magnet systems, strip-shaped ferromagnetic shims have been arranged on respective facing sides of a pair of magnet bodies each having a cylindrical hollow, and ring-shaped ferromagnetic shims have been arranged on both the facing sides and on the surfaces of the hollow portions, of the magnet bodies (e.g., Japanese Laid-Open Patent Publication No. 2002-336215).